This invention relates to the reproduction of photographs and more particularly, it concerns a method and apparatus for making photographic copies of transparencies and prints using conventional photographic materials.
Photographs of natural objects are, at best, approximations of tonal qualities visible in such objects due to the limited capability of photographic materials to respond accurately and completely to all naturally occurring light. This limited capability of photographic materials may be appreciated by noting that accuracy of tone reproduction, whether of shades of grey in black and white photography, or of saturation levels of various colors in color photography, will depend, among other factors, primarily on "contrastiness" of the photographic material. Because of this, most all of the presently available general purpose photographic films are designed with some compromise between tone reproduction capacity and contrastiness to provide a pleasing effect to one viewing the photographic image recorded on the film. Color films and print materials are especially notable in this respect, particularly those which are designed to enhance the brilliance of naturally occurring colors.
Characteristics of photographic materials, such as sensitivity or speed, contrast, exposure latitude and tone reproduction are conventionally represented by Hurter and Driffield (H & D) curves in which density (transmission or reflection density) is plotted against the logarithm of exposure. "Density", in context, is defined either as the logarithm of opacity for transparencies or, for prints, as the logarithm 1/R, where R, the reflectance, is the ratio of light intensity reflected by the print to that falling on it. The H & D curve for a given photographic material has a "slope" which is representative of contrast and tone reproduction. Specifically, a slope of 1:1 or "1" is a theoretical optimum for accuracy of contrast and tone reproduction of light reflected from a naturally occurring object or scene. In practice, however, most general purpose photographic materials are designed such that a substantial part of the H & D curve has a slope significantly greater than 1, for example, 1.5:1 and often exceeding 2 (2:1). The increased slope is necessary to record the extensive light range occurring in nature on film having a limited light and dark range, and results in enhanced color saturation, for example, in color photography and provides a reproduction more pleasing to the average observer than one which is not so enhanced.
Although the recognition and use of the aforementioned characteristics of photographic materials has contributed to the highly refined state of the art now available for general purpose photography, the H & D characteristic curve relied on to provide enhanced photographic images, presents a problem to the use of general purpose material for copying existing photographically recorded images. With the development of instant photography using well-known diffusion transfer processes in which the positive print is obtained without the separate step of making an internegative, for example, attempts have been made to use such materials for direct copying of images either by exposure to light projected through slide transparencies or reflected from positive prints. The results obtainable heretofore, however, have been inferior because of excess constrast in the copy as compared to the original or to copies made with special copy film material. The excess contrast is known to be a result of the effective multiplication of the H & D characteristic curve slopes of both the original and copy materials in the copying process. For example, a slide or print with a constrast value represented by a slop of 2, when copied with a film having a similar slope, will result in a print copy with a constrast represented in an H & D curve slope of 4. For this reason, prior attempts at using general purpose films for copying slide transparencies or prints have been abandoned in favor of special purpose films or other photographic materials for which the H & D characteristic curve slope is 1, or less than 1, in order that the copy will be a pleasing reproduction of the original photograph.
In light of the foregoing, one having only a modest familiarity with color photography will appreciate the desirablility of a facility by which conventional photographic materials could be used to provide accurate reproductions of existing photographically recorded images. In particular, the use of existing diffusion transfer films and the ease with which they can be processed or developed exemplifies the magnitude of advance represented by such a facility over presently available dark room techniques for making color prints from slides, for example.